component placement in the case

[Keyoke]

Gurus,

Is there a general 'rule of thumb' when placing the torroid, PSU and amp boards in the same chassis when it comes to placement?

I'm wondering if this works like premise wiring - "signal cables should cross power or other sources of EMI @ 90 degree angles, and should be run 18" apart" kind of thing.

I've seen a number of designs where the PSU/Torroid is seperated by a strip of metal or in an 'internal box' of some sort.

Can anyone offer suggestions? I'm trying to keep the size of the chassis down as much as I can w/o using an external case for the PSU/Torroid combo.

[DJ Exprice]

Just don't let AC wires mingle near and or with input wires or other sensitive places on the amp. And try to keep the transformer as far away as possible from the rest of the amp components.

[v-bro]

It's only an issue when you use components with stray magnetic fields.
It's not even a problem when the magnetic field cannot induct onto something, or even inducts onto something that isn't influenced by this.

Toroids are said to have minimal stray magnetic fields, so most of the times it's not a problem...Some manufacturers place ground planes out of precaution...

I'm no expert in this field, but I was allready thinking of a simple way to measure it (iron dust on a piece of paper?)...

indeed, Guru's?

[falcott]

If one is forced into positioning the transformer close to the amp, maybe he can use some steel shielding between them? If so, does the thickness of the steel make any difference?

[Nordic]

The large square EI frame transformer has a large but not terribly strong magnetic field coming out the curved "bell" ends of the transformer in broad circles with the transformer windings (under the "bells" that cover the windings) acting as the anchor point. There is little or no field radiated by the fat metal laminations. You do need to be careful where the magnetic field of the EI frame transformer is aimed. You want to keep it away from the low-level circuit boards and from the input RCAs and wiring.

The toroid, a round donut-looking transformer, has almost no magnetic field radiating out from it except along an almost straight line right through the hole in the center of the toroid. Within that center hole and extending up and down (toroids are almost always mounted flat because of this magnetic field) from it is one massively strong magnetic field. It has the same relative strength as the weaker but larger-sized field that radiates from the EI frame transformer, but it is so concentrated by the shape of the toroid transformer that that the density of the flux energy in the field radiating from a toroid is very high indeed. The larger the toroid, the stronger this radiated field. Imagine a miniature javelin going through the hole in the toroid transformer; this is a pretty good representation of the radiation pattern of the magnetic field (though the field spreads out a bit as you get farther away from the toroid). Placing some other component above or below an amplifier with a large toroidal transformer inside is asking for trouble. You could get hum or otherwise degrade the sound. Only a large amount of ferrous material, like cast iron, above and below a toroidal power transformer will block the magnetic field. The aluminum or thin steel chassis in most audio components is virtually transparent to the magnetic field from the transformer. So are wood or glass rack shelves. Even the second chassis of another nearby component won’t reduce the strength of the magnetic field too much. Other than an inch-thick cast iron plate, the best defense against these strong magnetic fields from toroidal transformers is distance or placing other components beside the component with the toroidal power transformer rather than above or below it.

Even the small toroid in a turntable or preamp power supply could induce hum in other components if they get to close to the javelin-like magnetic field radiating out from the hole in the center of the toroid.

Doug Blackburn


http://www.soundstage.com/maxdb/maxdb091998.htm

[CarlosT]

I'm thinking about doing a 3M copper tape "bellyband" for my little toroid since it's like 2 inches from the amp board. I saw this on the Bicron website (a manufacturer of toroids). Any thoughts on the efficacy of this?

[I_Forgot]

Originally posted by Nordic


Doug Blackburn
...

Something doesn't seem right about this stuff about the magnetic field along the axis of a toroid. Magnetic flux is perpendicular to the current through the coils that produce it (the good old right hand rule). That would do exactly what we expect and send the flux through the loop of the core.

There will be some axial flux because the coils of wire that wrap around the core essentially form a "super" coil of the same loop size as the core of the transformer (assuming they go all the way around the core radially), but they form only a 1 or 2 (maybe 3) turn loop. The permeability of the air is small compared to the core, and the opening is relatively large, so I'm pretty sure you won't find a "strong" field axial to the transformer core.

If someone has an EM field solver or a gauss meter we can find out about this real fast.

Anyway, it's best to keep low level signals and high impedance circuits away from high current parts like power transformers, so the transformer usually goes at one end of your box and the circuits at the other.

I_F

[diarav]

Originally posted by Keyoke
I've seen a number of designs where the PSU/Torroid is seperated by a strip of metal or in an 'internal box' of some sort.

Originally posted by falcott

If one is forced into positioning the transformer close to the amp, maybe he can use some steel shielding between them? If so, does the thickness of the steel make any difference?

Any thoughts on how effective the above would be? I am planning to implement something similar to be above for my PSU. I may have situations where the DC power wires run along the same line/space with signal. Planning to separate the DC wires with a thin steel plate.

Any opinions?

Thanks

[v-bro]

How does it operate without box, maybe you can test the audible influence by putting the transformer in the desired distance and further away... It should produce 50hz hum, shouldn't it?

[v-bro]

By the way, I just took a large air-core speaker filter coil and set my DMM on Ac mV, if I hold it near a PSU tranny it measuers a couple of mV, there's a simple Gauss meter!